I used to have ADT (came with the house). Then I unsubscribed from their service and I am now using EyezOn. I have the Vista 20P Hooked up onto an AlarmDecoder device. This device interfaces with the Alarm panel using a good ole RS232, so anything happening to the alarm, gets fed to my OH through the AlarmDecoder module. There is an AlarmDecoder Binding for OH that I have been using for a long time now. Any sensor you have hooked up on the Alarm gets fed to the binding using the Alarm Decoder Protocol, which means you can do anything with the sensors too!
Also, with this merged changes ( [AlarmDecoder] Added support for AlarmDecoder LRR Protocol by LuckyMallari · Pull Request #5019 · openhab/openhab1-addons · GitHub ) you can also determine what your Alarm is sending to the monitoring company.
I use all my Vista20P (alarm) sensors with my OH.